Lakatos found that many research programmes coexisted. Each had a hard core of theories immune to revision, surrounded by a protective belt of malleable theories.[2] A research programme vies against others to be most progressive.[2] Extending the research programme's theories into new domains is theoretical progress, and experimentally corroborating such is empirical progress, always refusing falsification of the research programme's hard core.[2] A research programme might degenerate—lose progressiveness—but later return to progressiveness.[2]